Silicon photonic integrated circuits (Si PIC) have attracted tremendous attention due to their high density integration capabilities of coupler, modulator, photodiode, avalanche photodiode, polarization diversity components and multimode interferometers. However, link budget deficiency has limited the application of Si PIC. In one aspect, the coupling efficiency of distributed feedback lasers (DFB-LD) and Si PIC has major influence on the link budget.
Up to the present time, there are two coupling structures adopted in Si PICs. The first coupling structure is grating coupler, which is favored for its on-chip characterizing compatibilities and high coupling efficiency with less than 2 dB insertion loss. However, one disadvantage of grating coupler is its limited spectral bandwidth, and the typical bandwidth within 1 dB variation is less than 20 nm. Another less-apparent but significant disadvantage is that its vertical packaging structure cannot be easily designed into QSFP28 form factors which have been the main stream packaging form factors in data center applications.
The second coupling structure is edge coupler, usually with an inverse tapered silicon waveguide surrounded by a SiO2 cantilever structure. Edge couplers are in-plane structures which can be fully compatible with QSFP form factors. Edge couplers may provide less than 2 dB insertion loss to single mode fiber. Edge couplers may also demonstrate coupling tolerance comparable with single mode fiber and very wide spectral bandwidth supporting O-band and C-band simultaneously. Nevertheless, the coupling loss of a DFB-LD is still beyond the acceptable range, due to the mismatch of DFB-LD mode size and edge coupler mode size. Lenses have been adopted to overcome the mismatch between two modes. However, most commercially available lenses are not designed for the coupling between DFB-LD and Si PIC edge coupler, as shown in FIG. 4, but are designed for coupling between DFB-LD and single mode fiber. In FIG. 4, the conventional coupling scheme 400 simply utilizes a lens to couple an output optical beam into an edge coupler of a Si PIC. Thus, it is not easy to find a suitable lens to optimize the coupling between DFB-LD and Si PIC edge coupler.